The present invention relates to an improvement of an outboard engine and, more particularly, to an improvement of a small to medium size outboard engine for use in fishing boats.
An outboard engine of small horsepower to be mounted on a fishing boat is generally required to be compact in size and light in weight for easier handling as well as to run at a high speed so as to arrive at the fishing area quickly. The outboard engine for fishing boats frequently used in rivers or lakes might be required, in addition to the above requisites, to sail the boat at such a low speed as 2 to 3 km per hour while trolling fishing lines behind thereof, or in other words, to move in what is called the trolling sail, depending on kinds of fish to be caught.
The outboard engine for fishing boats of the type mentioned above is often constructed as the direct-coupled type wherein the revolution of an engine is directly transmitted to a propeller without a clutch in order to build light weight engines. In such an outboard engine having a small horsepower as this, the revolution of the propeller generally is reduced to about one half of the revolution of a driving shaft. In the prior art outboard engine of the direct-coupled type, therefore, even when the engine is run at an idling rate of about 2,000 r.p.m. for the trolling sail, the revolution of the propeller becomes above 1,000 r.p.m. to make a boat as small as to accommodate two persons run at a speed higher than 4 km per hour, thereby presenting a problem when not running the boat at the low-speed trolling sail.
There has been known, another type of outboard engine which is provided with an automatic clutch, for instance a centrifugal clutch, between the crankshaft of the engine and the propeller driving shaft so as not to necessitate stopping of the engine even when the boat stands still. However, such conventional outboard engine with a clutch has drawbacks since the function of the clutch provided in the outboard engine mentioned above is so designed that it does not transmit the revolutional torque to the propeller driving shaft until the revolution of the engine reaches close to the rated revolution, for instance 3,500 r.p.m. During the idling operation period after starting the engine and before reaching near 3,500 r.p.m. in the engine of such a design, therefore, the centrifugal force on the clutch shoe is not sufficient to permit the engine revolution to be transmitted to the driving shaft. Generally, in a small engine of about 1 to 10 horsepower which is frequently utilized for a small boat built for two fishermen, when the centrifugal clutch mechanism reaches the "engaged" state at or around 3,500 r.p.m., the speed of the boat becomes 6 to 7 km per hour or higher. Accordingly, a boat, even if provided with a prior art centrifugal clutch in the outboard engine thereof, can not be run at a low-trolling speed simply by controlling a throttle thereof because the speed of the boat becomes too fast at a revolution which maintains the "engaged" state to allow the transmission to the propeller and if the revolution is further reduced to the "idling" state, the clutch becomes disengaged to cut completely the transmission to the propeller.